Dislocation creep of glaucophane in mafic blueschists during subduction: Weighted Burgers vector analysis from the Catalina Schist (California, USA)

Abstract Mafic blueschist is ubiquitous along the plate interface from the base of the seismogenic zone to blueschist-to-eclogite transition depths in many subduction zones. Currently, the deformation mechanisms of the rheology-controlling mineral glaucophane in blueschists are controversial. Dislocation creep, diffusion creep, and semi-brittle deformation have all been proposed, with specific implications for each in modulating slip behavior. We document the preserved deformation mechanisms in a lawsonite blueschist that equilibrated at ~300 °C and 1.0 GPa in the Catalina Schist on Pimu’nga (Santa Catalina Island, California, USA) through a novel method coupling weighted Burgers vector (WBV) and misorientation analyses of glaucophane. Electron backscattered diffraction–based analyses reveal strong crystallographic and shape preferred orientations in glaucophane and dislocations organized into subgrain boundaries. Misorientation distributions further illustrate dislocation-accommodated deformation and dynamic recrystallization by subgrain rotation (SGR). Subgrain boundaries show WBVs composed primarily of <100>, <010>, and <110> components, with lower representation of “easy” slip on <001> WBVs. Misorientation analysis demonstrates mother-daughter relationships between elongated core grains and recrystallized grains. Limited chemical zoning along microfractures suggests subsidiary diffusion creep during exhumation. Our results clearly document blueschist deformation primarily accommodated by dislocation creep and SGR recrystallization at pressure-temperature conditions of the base of the seismogenic zone.